The random error of the machine tool is caused by external conditions, and is greatly affected by the external conditions.
It can be divided into: positioning accuracy error, geometric accuracy error, thermal deformation error, etc. Let's briefly understand these three errors.
1. Error caused by load deformation of the process system: When turning, the workpiece is often affected by cutting force, clamping force, inertial force, gravity, etc., which will produce corresponding deformation, and ultimately destroy the correct relative position between the tool and the workpiece, reducing the machining precision of the workpiece. For example, when the rigidity of the workpiece is far less than the rigidity of the tool and fixture, the workpiece will be deformed due to insufficient rigidity under the action of the cutting force, thus reducing the machining accuracy. On the contrary, when the rigidity of the workpiece is far greater than the rigidity of the tool and fixture, the tool and fixture will be deformed during the processing of the workpiece, which will also reduce the accuracy of the workpiece.
Therefore, it is necessary to reasonably select the tool material, increase the rake angle and main deflection angle of the tool, and reasonably heat treat the workpiece material to improve its machining performance. At the same time, it is necessary to improve the rigidity of the process system, reduce the cutting force and compress their variation amplitude.
2. Error caused by redistribution of internal stress: the so-called internal stress is the stress that exists inside the part without the effect of external force. Once internal stress is generated on the workpiece, it will make the workpiece in an unstable state of high energy level, thus instinctively transforming to a stable state of low energy level, and with the deformation of the workpiece, the workpiece will eventually lose its original machining accuracy. For example, after heat treatment, internal stress is generated due to uneven wall thickness and uneven cooling of workpieces, which leads to deformation and ultimately reduces machining accuracy.
Therefore, when designing parts, we should try to achieve uniform wall thickness and symmetrical structure to reduce the generation of internal stress.
3. Error caused by thermal deformation: In precision machining and large piece machining, the thermal deformation of the process system has a great impact on the machining accuracy of the workpiece, and the machining error caused by thermal deformation can sometimes account for 40%~70% of the total error of the workpiece. Machine tools, cutting tools and workpieces are affected by various heat sources, and the temperature will gradually rise. At the same time, they transfer heat to the surrounding materials and space. Thus, the workpiece and the whole process system will be deformed. When the heat input per unit time is equal to the heat released, the process system will reach the thermal equilibrium state. For example, in the process of turning large parts, the temperature of the spindle bearing and the Z-axis lead screw increases due to excessive cutting force, resulting in large deformation, which affects the machining accuracy of the workpiece.
